The two most commonly used frequency bands set aside for cell phone use, are the AMPS band which extends from 806 to 894 MHz and which is sometimes referred to as the xe2x80x9c800 megahertz bandxe2x80x9d, and the PCS band which extends from 1850 to 1990 MHz and which is sometimes referred to as the xe2x80x9c1900 megahertz bandxe2x80x9d. The center of the lower frequencies is about 850 MHz while the center of the higher frequencies is about 1920 MHz. Cell phones are often used in vehicles, where much of the signal is lost due to the metal vehicle body. The losses can be greatly reduced by mounting an antenna outside the vehicle and coupling a cell phone to that antenna.
Antennas are available that are resonant to either the low frequency band of about 850 MHz (35.3 centimeters) or to the high frequency band of about 1920 MHz (15.6 centimeters). It is possible to mount two antennas, but this adds cost and complexity and cell phone users often do not know what frequency their cell phones operate on. Thus, there is a need for a cell phone antenna that can efficiently radiate at both the lower frequency of about 850 MHz and the higher frequency of about 1920 MHz, so it can be used with any of the latest common cell phones.
In accordance with one embodiment of the present invention, a cell phone antenna is provided, which can efficiency radiate at both common cell phone frequencies, of about 850 MHz and 1920 MHz. The antenna includes a vertically elongated antenna conductor that is divided into radiators that resonate at selected ones of the frequencies, and that include phase reversal means for producing 180xc2x0 phase reversals so two radiators spaced along the height of the antenna, that radiate at the same frequency are in phase for efficient combined radiation.
An upper conductor portion has a length that is about xc2xd wavelength (electrical) at the 850 MHz band to radiate at that frequency. An upper PRD (phase reversal device), with its upper end shorted and its lower end non-shorted, divides the upper conductor portion into lower and upper radiators that radiate at the 1920 MHz band. The upper PRD has a physical length of about xc2xc wavelength at the 1920 MHz band, to produce a phase reversal at the upper half of the upper conductor portion, so the upper and lower radiators at the 1920 MHz band radiate effectively. The upper PRD has no effect at 850 MHz, unlike other possible PRDs such as coils.
The lower conductor portion includes a coil that produces a phase reversal at the 850 MHz band. The antenna conductor forms a vertical wire that extends up from the top of a lower PRD to the bottom of the coil. The distance between a ground plane at the bottom of the lower conductor portion and the bottom of the coil is about xc2xc wavelength at the 850 MHz band to produce another low frequency radiator at that band. The lower PRD adds a phase reversal at its non-shorted top, at the 1920 MHz band. This allows the PRD to lie along the 850 MHz radiator without affecting the phase or electrical length of the 850 MHz radiator. The distance between the ground plane and the top of lower PRD is electrically xe2x85x9c wavelength at 1920 MHz, to provide a moderately effective impedance match for moderately efficient feeding of currents at 1920 MHz. The distance between the ground plane and the bottom of the coil is electrically xc2xc wavelength at 850 MHz, for efficient feeding at 850 MHz.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.